Diffractive lenses typically utilize diffractive “zones” that break up an optical wavefront with discontinuities. The lateral separations between the zone boundaries, and the optical phase delays at the zone boundaries, combine together to redirect the light in a controlled manner. The optical wavefront itself is typically discontinuous just after passing through a diffractive lens, unlike the situation for a conventional optical imaging system where the wavefront is typically smooth and continuous.
The optical wavefront of a conventional monofocal imaging system can be used to determine the optical properties of an image created by the system. The wavefront can be used to calculate the point spread function, the modulation transfer function, or a variety of other measures of image quality. One method that can be used to measure the wavefront of a conventional lens is a Shack-Hartmann system, where the wavefront illuminates an array of small lenslets. The light that passes through each lenslet comes to a focus, and if the local wavefront is tilted, the focused spot is displaced laterally by a distance that represents the local slope of the lens over the region of the lenslet. The slopes of the wavefront are measured for all the lenslets in this manner, and the slopes are combined to create the wavefront. This method has been used in many fields, and it has recently become popular in ophthalmology, where it can be used to measure the wavefront quality of the human eye.
Problems arise when a lenslet array is used to measure a diffractive lens, because the method can only measure local wavefront slopes, and it does not measure the zonal optical discontinuities that are a feature of diffractive lenses. A similar limitation exists for a Fresnel lens, which is a monofocal lens where the physical bulk of the lens is reduced by shifting the lens surface in the axial direction. These shifts can be at arbitrary locations, and they can have arbitrary optical phase delays for a Fresnel lens. The surface slope of the lens at any location is similar to the surface slope of the original lens, but phase discontinuities have been introduced that affect the optical properties.
One particular ophthalmic use of diffractive lenses is as an intraocular lens. Intraocular lenses (“IOLs”) are routinely implanted in patients' eyes during cataract surgery to compensate for the lost optical power that results when the natural lens is removed. The terms “intraocular lens” and its abbreviation IOL are used interchangeably herein to describe lenses that are implanted into the interior of an eye to either replace the natural lens or to otherwise augment vision regardless of whether or not the natural lens is removed. They provide an optical power for correcting a refractive error of the natural eye. Many different types of intraocular lenses exist for treating a variety of conditions to provide a patient with corrected vision.
Diffractive lenses can diffract light simultaneously into several directions, also typically known as diffraction orders. In multifocal intraocular lenses, two diffraction orders can be utilized to provide a patient with two optical powers: one for distance vision and one for near vision. Such diffractive Intraocular lenses are typically designed to have an “add” power that provides a separation between the far focus and the near focus. In this manner, a diffractive IOL can provide a patient with vision over a range of object distances.
When a diffractive IOL is implanted into an eye it affects the wavefront in the manner described above. Measurements using a lenslet array are affected by the discontinuities in the wavefront.